1. Field of the Invention
The present invention relates to a heater honeycomb having resistance adjusting means.
Honeycomb heaters of the above-described type can be employed as heaters for domestic use, such as hot-air heaters, or as industrial heaters, such as preheaters used for control of automobile exhaust emissions.
2. Description of the Prior Art
Recently, besides conventional porous ceramic honeycomb structures, metal honeycomb structures have drawn attention as catalysts or catalyst carriers for removing nitrogen oxide, carbon monoxide and hydrocarbons present in the exhaust gas from internal combustion engines, such as those in automobiles.
As restriction of exhaust emissions has increased, there has been a demand for development of heaters for use in reducing automobile exhaust emissions when the temperature of the catalyst is not sufficiently high, i.e., at the beginning of running of a vehicle.
Honeycomb structures have been proposed in, for example, U.K. Patent 1492929 and Japanese Utility Model Laid-Open No. 67609/1988.
U.K. Patent 1492929 discloses a foil-type metal honeycomb structure in a catalyst for control of automobile exhaust emissions. This honeycomb structure comprises a metal substrate produced by winding a flat plate and a mechanically deformed, corrugated plate together. This metal substrate has an aluminum oxide film formed on the surface thereof by oxidation. The catalyst for use in automobile exhaust emission control is manufactured by placing a large surface area oxide, such as alumina, on the aluminum oxide film of the metal substrate and by impregnating a noble metal on the large surface area oxide.
Japanese Utility Model Laid-Open No. 67609/1988 discloses an electrically conductive metal monolith preheater including a metal support and alumina coated thereon.
In the foil-type metal honeycomb structure disclosed in U.K. Patent 1492929, however, the metal substrate with a coating formed thereon does not closely adhere to a catalyst layer because of low porosity of the metal substrate, and a ceramic catalyst readily peels from the metal substrate due to a difference in the thermal expansion between the ceramic catalyst and the metal substrate. Furthermore, telescope phenomenon readily occurs during the run cycle in which a metal-to-metal join breaks and the metal substrate is deformed in such a manner that it protrudes in the direction of the flow of gas. This may disturb safe running of the vehicle. Furthermore, in the manufacture of the foil-type metal honeycomb, yield of the rolling process is low, inviting high production cost. In the preheater proposed in Japanese Utility Model Laid-Open No. 67609/1988, a catalyst readily peels from due to a difference in thermal expansion between alumina and the metal support. Furthermore, a metal-to-metal join of the metal substrate breaks during the operation, generating an electrically insulating portion and, hence, non-uniform flow of current and non-uniform heating.
The preheater disclosed in Japanese Utility Model Laid-Open Open No. 67609/1988 is constructed so that a current is supplied between the inner periphery and the outer periphery of the foil-type metal honeycomb structure to generate heat. However, the preheater is not arranged so that it has an adjusted resistance (i.e., the material, dimension and rib thickness of the honeycomb structure are fixed and resistance cannot be adjusted to that desired), and therefore, exhibits insufficient temperature raising characteristics. Furthermore, since the electrodes are provided on the inner peripheral portion of the preheater, the central portion thereof does not act as a catalyst and pressure loss may be generated. Furthermore, the electrodes readily from the heater due to the flow of gas.